1. Field of the Invention
The present invention relates generally to a film forming method, and more particularly, to a method for forming a film of uniform thickness on a semiconductor substrate having a concave portion.
2. Description of the Background Art
In the manufacturing process of a semiconductor device, fine shapes such as a transistor and an interconnection are formed on a semiconductor substrate through complex processes including a plurality of times of the photolithographic process. A complex irregular shape can be formed on the substrate through such processes. In the subsequent photolithographic process, nonuniformity of thickness of a photoresist film due to such an irregular shape is caused in applying a photoresist material by spin-coating. In particular, a scribe line which is a trench for isolating a plurality of chips formed on the substrate presents irregularity larger in scale than fine irregularity inside of a chip, so that nonuniformity of thickness of the resist film caused by the irregularity becomes larger.
FIG. 1A to 1D are cross-sectional views showing manufacturing processes of a semiconductor device in a film forming method which is the background of the present invention. As shown in FIG. 1A, a scribe line 3 is provided on a semiconductor substrate 1 for isolating chips 2. The width of the scribe line 3 is 80 to 150 .mu.m and the depth thereof is 1 to 5 .mu.m.
When a photoresist of, for example, a positive type is directly applied to this semiconductor substrate 1, e.g., by spin-coating, the thickness of the applied photoresist 4 becomes thinner in the vicinity of the scribe line 3 than that in another portion, as shown in FIG. 1B. When a predetermined region of such a photoresist of non-uniform thickness is irradiated with light (represented by an arrow) using a photomask 5 having patterns 51 of the same line width as shown in FIG. 1C and then developed, a photoresist pattern 6a in the vicinity of the scribe line 3 becomes thinner and narrower than a photoresist pattern 6b far away therefrom.
FIGS. 2A to 2B are cross-sectional views showing other manufacturing processes of a semiconductor device in the film forming method which is the background of the invention, showing a case in which there is formed a pattern of an on-chip type color filter material for a solid state imager. As shown in FIG. 2A, a lot of photodiodes 7 are arranged in a line manner or an array manner on chips 2 isolated from each other by a scribe line 3 on a semiconductor substrate 1. An oxide film 8 for isolation is formed between the adjacent two photodiodes 7. A material known to be suitable for such a film 8 is silicon diode. The photodiodes 7 are arranged closer to the scribe line 3. When a negative type photosensitized gelatin is applied on such a semiconductor substrate 1 as a color filter material, the thickness of a photosensitized gelatin film 9 generally becomes thinner in the vicinity of the scribe line, as shown in FIG. 2B. Then, when predetermined regions of the gelatin photosensitive film 9, for example, regions on the photodiodes 7 are selectively irradiated with light using a photomask 5 as shown in FIG. 2C and then developed, a gelatin pattern 10a in the vicinity of the scribe line 3 becomes thinner than a gelatin pattern 10b far away therefrom, as shown in FIG. 2D. Thereafter, each gelatin pattern is subjected to dyeing processing. There occurs a difference in spectral transmittance characteristics between the dyed gelatin pattern 10a in the vicinity of the scribe line 3 and the dyed gelatin pattern 10b far away therefrom even if the gelatin patterns are respectively color filters of the same color. Thus, there occurs a difference between respective spectral response of the photodiodes 7 corresponding to the gelatin patterns 10a and 10b.
In the above described two examples, the thickness of the gelatin photosensitive film in the vicinity of the scribe line is thinner than that in another portion. However, a coating material may protrude on a side of the scribe line at the time of application by spinning depending on the nature of the material to be coated, for example, the viscosity, the surface tension and the like. Such a phenomenon is described in an article by Ikeno et. al., entitled "Effects of Superficial Topography on Uniformity of Spun-on Resist Film", EXTENDED ABSTRACTS in Fall Meeting of The Electrochemical Society held in October, 1988 Vol. 88-2, pp. 1041-1042.
As described in the foregoing, in the conventional film forming method, a film of a photoresist, a color filter material or the like becomes thin or thick in the vicinity of a concave portion such as a scribe line on a semiconductor substrate. Thus, uniformity of the pattern size of the resist or the thickness of the film is decreased, so that a semiconductor device as designed cannot be obtained. In addition, spectral transmittance characteristics of a color filter film and spectral response of the photodiode become non-uniform depending on the position of the gelatin pattern on the chip. Thus, the characteristics of a solid state imager so formed are not entirely satisfactory.
In order to solve the above-described problems, the thickness of the film is made thick in practice so that the effect due to the difference in thickness is decreased. However, in such a case, the resolution in patterning the film is lowered and the reliability is decreased. In addition, much time is required for exposure and development.